1. Field of the Invention
The present invention relates to a structure using a photonic crystal and a surface emitting laser including the structure.
2. Description of the Related Art
Optics Express, vol. 13, No. 17, p. 6564 (2005) discusses guided resonance using a photonic crystal in a shape shown in FIG. 8.
The structure shown in FIG. 8 includes a sapphire substrate 8000, a photonic crystal layer 8050 including a GaN layer 8010 having periodically provided holes 8020, and an AlN nucleation layer 8030.
Guided resonance means that guided mode light, guided in a photonic crystal having a periodic refractive index structure in its in-plane direction, is coupled with a radiation mode and is thereby emitted from the photonic crystal.
A photonic crystal can be used as a mirror by use of guided resonance. More specifically, light coming into a photonic crystal in the direction perpendicular to the in-plane direction of the photonic crystal can be converted into light guided in the photonic crystal by being coupled with light modes above the light line. The converted light is coupled with radiation mode light and thus emitted to the outside from the photonic crystal. Light reflected directly without being coupled with the guided mode light interferes with the light emitted to the outside by coupling with the guided mode light and causes peculiar reflection, for example, reflection with an efficiency of 100%.
In general, the guided mode refers to a mode satisfying the requirement that light does not leak from a photonic crystal layer, and the radiation mode refers to a mode in which light leaks from the photonic crystal layer.
A light line shows a dispersion of light propagating in a medium adjoining a waveguide layer (in this instance, photonic crystal layer), and can be represented by a line defined by an expression w=ck/n (w: angular frequency; c: velocity of light; n: refractive index; k: wave number). In general, light in the region having higher frequencies than the light line can leak easily from the photonic crystal.
Thus, the operational principle of mirrors using guided resonance is different from that of mirrors using the photonic band gap.
If incident light comes perpendicularly into the photonic crystal layer shown in FIG. 8, and if that light is adjusted to have a wavelength in a region in which the reflectance is remarkably increased by guided resonance, the adjusted light is reflected with a high reflectance.
The above-cited Optics Express document discusses a simulation of how the guided resonance changes when the refractive index of the sapphire substrate 8000 adjoining the photonic crystal layer 8050 (refractive index of GaN: 2.37) varies.
FIG. 9 shows spectra (vertical axis: frequency, horizontal axis: transmittance) of guided resonance occurring in a photonic crystal layer when the refractive index (represented by n in FIG. 9) of the substrate is varied while the refractive index of the photonic crystal layer is fixed.
Also, FIG. 9 shows that as the relative refractive index difference (Δn=(nphc−nclad)/nphc) between the photonic crystal layer (refractive index: nphc) and the substrate (refractive index: nclad) acting as a clad layer is reduced, guided resonance becomes difficult to produce.
More specifically, when nphc=2.37 and nclad=1.8 (n represents nclad in FIG. 9), the relative refractive index difference is about 0.24 (about 24%). In this instance, guided resonance occurs as is clear from FIG. 9.
When nphc=2.37 and nclad=2.135, that is, when the relative refractive index difference is about 0.10 (about 10%) or less, guided resonance does not occur.
In order to apply a mirror using guided resonance in a photonic crystal to a laser, it may be required that the difference in refractive index between the photonic crystal layer and its adjoining clad layer be very small.
When, for example, a certain difference in refractive index is needed between a photonic crystal layer and its adjoining layer, air having a low refractive index can be used as the layer adjoining the photonic crystal layer. However, such a structure, a so-called air gap structure, is difficult to form by a semiconductor deposition process. Accordingly, if a photonic crystal is applied to an optical device, a structure is desired in which guided resonance can occur even though a semiconductor having a higher refractive index than air adjoins the photonic crystal layer.
When using a photonic crystal layer instead of the multilayer mirror of a surface emitting laser emitting light having a wavelength of 670 nm, a combination of an Al0.5Ga0.5As photonic crystal layer (refractive index: 3.446) and an Al0.93Ga0.07As clad layer (refractive index: 3.130) adjoining the photonic crystal layer may be proposed.
In this instance, the relative refractive index difference Δn (=(nphc−nclad)/nphc) is about 9.2%. Hence, guided resonance is difficult to produce in the structure described in the above-cited document, and it is difficult to use the photonic crystal as an alternative to the multilayer mirror.